1. Field of the Invention
The present invention relates to telecommunications networks and, more specifically, to application identification within digital service hierarchy level 3 networks.
2. Description of the Related Art
Digital service hierarchy level 3 (DS3) is a digital, dedicated, private-line service that provides high-capacity transport for combinations of data, voice, and video applications. DS3 is a North American standard developed and standardized by the American National Standards Institute (ANSI) T1 sub-committee. Relevant specifications include “Network and Customer Installation Interfaces—DS3 Metallic Interface Specification,” T1.404-2002 (herein “T1.104”) and “Digital Hierarchy—Formats Specifications”, T1.107-2002 (herein “T1.107”). Each specification is incorporated herein by reference in its entirety.
DS3 was initially designed to assist voice carriers in consolidating multiple, lower-speed voice connections into a single high-speed communication path between a data source and a data sink. Most carriers moved to DS3 technology in the late 1970s. Today, DS3 serves as the physical interface for a wide variety of point-to-point and site-to-site communication applications.
It should be noted that, in the literature and in this document, the terms “DSn” where n is one of {3, 2, 1, 0} will be used synonymously with the terms “Tn.” However, strictly speaking, DSn is actually the contract between a carrier and an end user at a particular level n of the digital service hierarchy, the contract defining a service level and cost, while Tn is a DSn signal that is sent over terrestrial lines.
DS3 is a multiplex of lower-speed digital services (namely levels 0, 1, and 2 of the digital service hierarchy). A DS3 is typically formed by multiplexing together seven digital service hierarchy level 2 (DS2/T2) services in an M23 multiplexor. Each of these DS2 services is, in turn, formed by multiplexing together four digital service hierarchy level 1 (DS1/T1) services in an M12 multiplexor. Generally, each DS1/T1service is formed by multiplexing together 24 DS0/T0 services. A DS0/T0 is a 64 kbps service typically used to carry a digital representation of a roughly 3 kHz analog voice signal sampled with an 8-bit companding A/D at 8 kHz.
Each frame (also known as an M-frame) of a DS3 service is composed of seven sub-frames (also known as M-subframes) corresponding to the seven DS2s that contribute to the DS3 multiplex. Among various overhead bits added to DS2 and DS3 frames, “stuffing” or “C-bits” are provided to deal with synchronization of the services at each level, given that the timing for the multiplexed services can potentially be based on clocks that are asynchronous to each other. There are twenty-one C-bits provided per M-frame. These are labeled Cij where iε {1,2,3},j ε {1,2,3,4,5,6,7}, where j corresponds to the M-subframe. So, for example, C11 (also known as application identification channel (AIC) signal) is the first C-bit of the first M-subframe and C37 is the third C-bit in the seventh M-subframe.
There are two primary applications for DS3 specified in T1-107. The first, termed M23, uses the C-bits as stuffing bits to accommodate synchronization between multiplexed streams as described above. The second, known as C-bit parity, assumes either that the incoming pseudo DS2 bitstreams are synchronous or that the DS3 is unchannelized data at 44.210 Mbit/s. In the C-bit parity case, the C-bits are not needed for stuffing purposes. They are thus utilized to improving management and performance functions. T1.107 (section 9.3.1) declares that the first C-bit in M-subframe 1 (the AIC signal) shall be set to 1 to identify the application of the DS3 service as C-bit parity. Thus, it is suggested by the standard that an endpoint (i.e., sink) for a DS3 link could use the state of this signal to detect the application; C-bit parity if the AIC signal is logical one, and M23 if the AIC signal is logical zero.
However, in an M23 application, there remains a small probability that the AIC signal will be set to logical one as alluded to in a footnote in T1.107 (section 9.3.1). This can lead to a false-positive indication of the C-bit parity application. Such a detection error might eventually be resolved, but only after delays in synchronization at the sink end of the DS3 stream.